Pilots of Asiana Flight 214 were flying too slowly as they approached San Francisco airport, triggering a warning that the jetliner could stall, and then tried to abort the landing seconds before crashing, according to federal safety officials.

The Boeing 777 was traveling at speeds well below the target landing speed of 137 knots per hour, or 157 mph, said National Transportation Safety Board chief Deborah Hersman at a briefing Sunday on the crash.

A Boeing 777 airplane lies burned on the runway after it crash landed at San Francisco International Airport July 6, 2013 in San Francisco, California. An Asiana Airlines passenger aircraft coming from Seoul, South Korea crashed while landing. There has been no official confirmation of casualties. (Photo by Ezra Shaw/Getty Images)

“We’re not talking about a few knots,” she said.

Hersman said the aircraft’s stick shaker – a piece of safety equipment that warns pilots of an impending stall – went off moments before the crash. The normal response to a stall warning is to increase speed to recover control.

There was an increase several seconds before the crash, she said, basing her comments on an evaluation of the cockpit voice and flight data recorders that contain hundreds of different types of information on what happened to the plane.

And at 1.5 seconds before impact, there was a call for an aborted landing, she said. The crash at San Francisco International Airport on Saturday killed two 16-year-old girls from China and injured dozens of others.

The new details helped shed light on the final moments of the airliner as the crew tried desperately to climb back into the sky, and confirmed what survivors and other witnesses said they saw: a slow-moving airliner.

Pilots normally try to land at the target speed, in this case 137 knots, plus an additional five more knots, said Bob Coffman, an American Airlines captain who has flown 777s. He said the briefing raises an important question: “Why was the plane going so slow?”

The plane’s Pratt & Whitney engines were on idle, Hersman said. The normal procedure in the Boeing 777, a wide-body jet, would be to use the autopilot and the throttle to provide power to the engine all the way through to landing, Coffman said.

There was no indication in the discussions between the pilots and the air traffic controllers that there were problems with the aircraft.

Among the questions investigators are trying to answer was what, if any, role the deactivation of a ground-based landing guidance system played in the crash. Such systems help pilots land, especially at airports like San Francisco where fog can make landing challenging.

Altogether, 305 of the 307 people aboard made it out alive in what survivors and rescuers described as nothing less than astonishing after a frightful scene of fire burning inside the fuselage, pieces of the aircraft scattered across the runway and people fleeing for their lives.

A Boeing 777 airplane lies burned on the runway after it crash landed at San Francisco International Airport July 6, 2013 in San Francisco, California. An Asiana Airlines passenger aircraft coming from Seoul, South Korea crashed while landing. There has been no official confirmation of casualties. (Photo by Ezra Shaw/Getty Images)

The flight originated in Shanghai, China, stopped over in Seoul, South Korea, before making the nearly 11-hour trip to San Francisco. The South Korea-based airline said four South Korean pilots were on board, three of whom were described as “skilled.”

Among the travelers were citizens of China, South Korean, the United States, Canada, India, Japan, Vietnam and France. There were at least 70 Chinese students and teachers heading to summer camps, according to Chinese authorities.

As the plane approached the runway under clear skies – a luxury at an airport and city known for intense fog – people in nearby communities could see the aircraft was flying low and swaying erratically from side to side.

On board, Fei Xiong, from China, was traveling to California so she could take her 8-year-old son to Disneyland. The pair was sitting in the back half of the plane. Xiong said her son sensed something was wrong.

On audio recordings from the air traffic tower, controllers told all pilots in other planes to stay put after the crash. “All runways are closed. Airport is closed. San Francisco tower,” said one controller.

At one point, the pilot of a United Airlines plane radioed.

“We see people … that need immediate attention,” the pilot said. “They are alive and walking around.”

“Think you said people are just walking outside the airplane right now?” the controller replied.

“Yes,” answered the pilot of United Flight 885. “Some people, it looks like, are struggling.”

When the plane hit the ground, oxygen masks dropped down, said Xu Da, a product manager at an Internet company in Hangzhou, China, who was sitting with his wife and teenage son near the back of the plane.

When he stood up, he said he could see sparking – perhaps from exposed electrical wires.

A piece of wreckage from Asiana Flight 214 after the plane crash landed at San Francisco International Airport on July 6, 2013. (Photo courtesy of the National Transportation Safety Board)

He turned and could see the tail where the galley was torn away, leaving a gaping hole through which they could see the runway. Once on the tarmac, they watched the plane catch fire, and firefighters hose it down.

“I just feel lucky,” said Xu, whose family suffered some cuts and have neck and back pain.

In the chaotic moments after the landing, when baggage was tumbling from the overhead bins onto passengers and people all around her were screaming, Wen Zhang grabbed her 4-year-old son, who hit the seat in front of him and broke his leg.

Spotting a hole at the back of the jumbo jet where the bathroom had been, she carried her boy to safety.

“I had no time to be scared,” she said.

At the wreckage, police officers were throwing utility knives up to crew members inside the burning wreckage so they could cut away passengers’ seat belts. Passengers jumped down emergency slides, escaping from billowing smoke that rose high above the bay.

Nearby, people who escaped were dousing themselves with water from the bay, possibly to cool burn injuries, authorities said.

A view of the interior of Asiana Flight 214 after the plane crash landed at San Francisco International Airport on July 6, 2013. (Photo courtesy of the National Transportation Safety Board)

By the time the flames were out, much of the top of the fuselage had burned away. Inside The tail section was gone, with pieces of it scattered across the beginning of the runway. One engine was gone, and the other was no longer on the wing.

San Francisco Fire Department Chief Joanne Hayes-White said the two 16-year-old girls from China who died were found on either side of the plane. Investigators are trying to determine whether they were alive or dead when rescuers reached the scene.

“What we saw yesterday, most people will never see in their career,” Hayes-White said.

Lowy reported from Washington, D.C. Associated Press writers Terry Collins, Terry Chea and Sudhin Thanawala in San Francisco, David Koenig in Dallas and Louise Watt in Beijing contributed to this report.